It is well-known that in the formation of an image by the conventional electrophotographic process, the resolution and quality of the image obtained in the process depend closely upon the particle size of the toner used in the process. That is, a finest possible particles-having toner needs to be used in order to obtain a high-resolution, finely detailed image. For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication No. 68047/1983 discloses a technique for the formation of an image by use of a toner containing more than 80% by weight particulate constituent whose particle size is not more than 10 .mu.m. Also. Japanese Patent O.P.I. Publication No. 181362/1984 discloses a technique for the formation of an image by developing an electrostatic image formed on a CdS photoreceptor in accordance with the non-contact developing process with use of a particulate toner whose particle size is not more than 10 .mu.m.
However, it is the present state that these image-forming methods using such particulate toners as mentioned above are unable to accomplish a practical reality because there is the following drawback to them:
That is, where such particulate toner is used, the toner tends to stick relatively fast to the surface of an image-forming member and, even when subjected to cleaning in the cleaning process, cannot be easily removed remains on the surface, thus deteriorating the electrophotographic characteristics of the image-forming member. The cause of such trouble is considered to be due to the fact that the smaller the particle size of the toner the more closely does the toner come into contact with and stick to the surface of the image-forming member where adhering force largely functions according to Van der Waals force and image force. etc., and further, part of the particulate toner passes under the cleaning blade so that it cannot be cleared off. Consequently, the residual toner covers all the surface of the image-forming member, thus deteriorating the characteristics thereof. Incidentally, those techniques for increasing the cleaning effect of such image-forming member have also been investigated.
For example. Japanese Patent O.P.I. Publication No. 176053/1985 discloses a technique to incorporate into a toner an amount of from 0.01 to 10% by weight metallic salts or a mixture thereof such as maleic acid metallic salts of zinc, magnesium, calcium. etc.; atearic acid metallic salts of zinc, cadmium, barium, lead, iron, nickel, cobalt, copper, aluminum, magnesium, etc.; dibasic lead stearate; oleic acid metallic salts of zinc, magnesium, iron, cobalt, copper, lead, calcium, etc.; palmitic acid metallic salts of aluminum, calcium, etc.; lead caprylate; lead caproate; linolic acid metallic salts of zinc, cobalt, etc.; calcium ricinolate; recinoleic acid metallic salts of zinc, cadmium, etc.; and the like. However, such techniques may be effective in those image-forming methods using a toner of a 10.mu. or larger particle size as described in the example of the above-mentioned publication, but, where an image is to be formed by using a developer comprised principally of a particulate toner having a particle size of not more than 10 .mu.m for the purpose of accomplishing such a high quality image as mentioned above, is unable to provide adequate effect in the cleaning.
On the other hand, realization of an image-forming member (hereinafter referred to as merely `photoreceptor`) excellent in the cleanability has also been investigated. For example, Japanese Patent Examined Publication No. 35059/1985 describes a technique in which, in place of the conventional Se photoreceptor, CdS photoreceptor and OPC photoreceptor, a nonpollution photoreceptor is used which has an amorphous silicon (hereinafter referred to as a-Si) photosensitive layer excellent in the light resistance, corona-ion resistance and heat/moisture resistance as well as in the mechanical wear resistance, having a high hardness (having a Vickers hardness of from 1,000 to 1,200 as compared to the Se photosensitive layer having a Vickers hardness of 60). and the surface of which is covered with a resinous protective layer such as of polyethylene terephthalate, polycarbonate, polyethylene fluoride or the like so as to be improved to be more moisture-resistant, wear-resistant and cleanable. In the photoreceptor described in the foregoing publication, the reason why the excellent mechanical wear resistance-having a-Si photosensitive layer further has a protective layer thereon is as follows:
The a-Si that constitutes the foregoing photosensitive layer has the disadvantage that since it has in itself a Si-Si bonding-cut dangling bond, it has a lot of localized levels inside the energy gap to cause the thermal-excitation carrier to make hopping conduction and therefore the dark resistance is small, and besides, the photo-excitation carrier is trapped by the foregoing localized levels, so that the carrier is not allowed to exhibit its photoconductivity. Upon this, in order to provide a satisfactory photoconductivity, the publication proposes the formation of a photosensitive layer of a construction of a-Si:H, a-Si:X or a-Si:H:X made by combining a hydrogen atom (H) and/or a halogen atom (X) to fill the gap caused by the dangling bond. Such the photosensitive layer is excellent in the photoconductivity, but is poor in the dark resistance: its resistivity is from 10.sup.8 to 10.sup.9 .OMEGA.cm, which is only one 10,000th of that of a-Se. Also, the photosensitive layer tends to become affected during a long period of time by the air, moisture or chemical species due to corona discharge, and its receptive potential can be significantly lowered.
Hereupon, the photoreceptor having the a-Si layer described in the foregoing publication is so constructed as to have thereon further the afore-mentioned surface-improving layer (protective layer) comprised of an insulating resin. In addition, Japanese Patent Application No. 225646/1985 which was applied for earlier by the same applicant discloses a technique to improve both the mechanical strength and the durability of a photoreceptor having an a-Si layer by covering the surface thereof with a surface-improving layer having a thickness of 400 A to 5,000 A comprised of an insulating inorganic material such as Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5, CeO.sub.2, ZrO.sub.2, TiO.sub.2, MgO, ZnO, PbO, MgF.sub.2, ZnS. or the like.
However, in the case where the formation of an image is made by using a particulate toner which is essential for accomplishing a high-quality image as mentioned above, the toner sticks so fast to the surface of the photoreceptor that the toner cannot be sufficiently removed by ordinary cleaning manner, thus necessarily requiring strong cleaning under a contact pressure several times as strong as that of the ordinary cleaning member. Thus, the resinous improving layer described in the foregoing Japanese Patent Examined Publication No. 35059/1985, when subjected to the above-mentioned strong cleaning, has its surface worn out and loses its improving function. Also, the above improving layer has the disadvantage that since it is an insulating layer, when the photoreceptor is exposed to light, its residual potential is large, tending to deteriorate the resulting image quality. The disadvantage of this kind may be found also in the photoreceptor having an improving layer comprised of the insulating inorganic material described in Japanese Patent Application No. 225646/1985.